This invention relates to a lip seal device with a lip which is slidably mounted on a rotary shaft.
One example of a conventional lip seal device comprises: a case surrounding a rotary shaft which is rotatably supported; a lip having an outer peripheral base end portion which is secured to the case, and an inner peripheral front end portion which is curved towards a sealed fluid and is in sliding contact with the outer surface of the rotary shaft; and an annular groove formed in the sealing surface of the lip which is in contact with the rotary shaft, the annular groove being a continuous-ring-shaped one which is perpendicular to the axis of the lip (Japanese Patent Application (OPI) No. 112082/1989 (the term "OPI" as used herein means an "unexamined published application")).
In another example of the conventional lip seal device, the outer peripheral base end portions of two lips are secured to a case, the inner peripheral front end portions of the two lips are curved towards the sealed fluid in such a manner that they are in slide contact with a rotary shaft, and the front end portion of one of the two lips, which is closer to the sealed fluid, is laid over the front end portion of the other lip to urge the latter towards the rotary shaft (Japanese Utility Patent Application (OPI) No. 20569/1988).
In the former lip seal device, the annular groove is formed in the lip as was described above; in other words, in the lip, the part having the annular groove is smaller in thickness than the remaining. Accordingly the part is readily deformed when pushed by the sealing fluid. As a result, two parts on both sides of the annular groove are more strongly pushed against the rotary shaft (hereinafter referred to as "contact parts", when applicable). Since the annular groove is continuous circumferentially of the lip, the contact parts are also each in the form of a continuous ring, thus providing a double seal action. Thus, the lip seal device provides excellent sealing effect not only when the rotary shaft is stopped but also when it is rotating.
However, the lip seal device suffers from the following difficulty: In general, the lip is made of a relatively hard material such as PTFE in order to improve its durability. Hence, when the lip seal device is greatly vibrated, or when the rotary shaft vibrates due to its eccentricity or play, the front end portion of the lip is raised from the rotary shaft being unable to follow the vibration, so that the fluid may leak out.
In the latter lip seal device, the double sealing action provides an excellent sealing effect. In the case where the lip closer to the sealed fluid is made of a relatively soft material such as soft rubber, it pushes the front end portion of the other lip against the rotary shaft. Hence, even if the other lip is made of a relatively hard material which is high in durability, it is prevented from being raised from the rotary shaft. Thus, even when the lip seal device is significantly vibrated, or when the rotary shaft vibrates do to its eccentricity or play, an excellent sealing effect is obtained.
However, the lip seal device is also disadvantageous as follows. Where the lip closer to the sealed fluid is made of a relatively soft material such as soft rubber, then its durability is lower, and it is liable to be damaged when subjected to high speed and high pressure. If the lip loses its sealing action, then the lip seal device effectively has only one lip, as a result of which it is difficult for the lip seal device to have a sufficiently high sealing effect.